The present invention relates to digital time bases, and more particularly to a digital time base with corrected analog interpolation having a total analog ramp delay exactly equal to a digital time delay.
In time domain reflectometers (TDR), such as the OF150 manufactured by Tektronix, Inc. of Beaverton, Oreg., United States of America, energy pulses are transmitted into a cable at a given pulse repetition rate depending upon the designed range of the TDR. Reflections from flaws, discontinuities or breaks in the cable are timed from the time of transmission of the energy pulse to determine the range from the transmitter of such flaw, discontinuity or break. To determine the range a clock provides a plurality of range pulses between energy pulses. The time interval between the clock pulses is a coarse determination of time which is a measure of range, i.e., for a 20 MHz clock the coarse time interval is to the nearest 50 nsec. To obtain finer time resolution an analog ramp is generated between clock pulses starting at a zero value at one pulse and terminating at a maximum value at the next pulse, i.e., the analog ramp goes from zero to maximum in 50 nsec. The ramp is sampled by comparing the ramp with a comparator voltage level which is set to occur when the reflection is received. The value of the ramp when the reflection is received determines the delta time, and therefore the delta range, between the clock pulses to be added to the coarse range as determined by the summation of the clock pulses occurring prior to the reflection receipt. If the ramp slope, due to aging of components, temperature variations or other effects, is not precise, the ramp may reach the maximum value either prior to or after the next pulse, resulting in a distortion in the range of the reflected signal being observed on the TDR display.
Therefore what is desired is a means for calibrating the analog ramp so that the time difference between the zero and maximum values is essentially equal to the digital time interval between clock pulses.